Check dead letter queues in system tests

[patchwork01]

Background

Currently system tests pass or fail based on assertions on the state of Sleeper tables.

Description

We'd like system tests to fail if there are any messages on dead letter queues at the end of the tests.

If the messages are related to other tables than the ones created for the tests, we'd like to ignore them. In case other tests are running in parallel, we'd like to ensure the test that did create those tables will be able to find the dead letter message as well.

Analysis

We could have a store that's shared by all tests that will receive messages from all the dead letter queues and cache them in memory to fail the relevant tests at the end.

We can have the JUnit extension SleeperSystemTestExtension check for any dead letters relevant to the current test in afterEach. If any dead letters are relevant to an instance as a whole rather than specific tables, we could fail every test that was connected to that instance at the time the dead letter was found. We'd need to add a thread-safe way to track the current running tests.